The present invention generally relates to satellite communication systems. In particular, the present invention relates to controlling the uplink power in a satellite communication system.
Satellites have long been used to provide communication services to large regions of the globe. Historically, communication satellites have used frequencies in the rangexe2x80x94of 3 to 12 GHz (C or Ku band) to produce an antenna beam which covers a large portion of a continent. Modern satellites may operate at frequencies of 20 to 30 GHz (Ka band) to produce a beam which may cover an area (or xe2x80x9ccellxe2x80x9d) with a diameter of 300 to 400 miles. Many such cells may be needed to provide communications to a region which previously may have been serviced by a single antenna beam. A modern cellular communication satellite may employ many antennas to generate a large number of beams used for transmitting downlink signals to, and receiving uplink signals from, various User Earth Terminals (UET) distributed over the surface of the earth.
In order for communication to occur on the uplink, signals generated by the UET must be of sufficient power to be received by the satellite. Thus, the antenna gain of the satellite""s uplink antenna coupled with the transmission power of the UETs must be sufficient to allow communication to occur. Typically, communication satellite systems are designed with a predetermined, fixed satellite uplink antenna gain. Thus, the transmission power of the UET is typically controlled to enable and ensure communication.
In practice, several factors exists which may negatively impact the uplink communication channel. That is, certain undesired influences may cause the actual antenna gain to vary from the predetermined, designed antenna gain or may cause attenuation of, or interference with, a signal transmitted by a UET. For example, antenna gain may be affected by gain roll-off which may cause the antenna gain to vary spatially over the cell or, alternatively, antenna gain may vary over the cell as a result of pointing errors in the antenna. Atmospheric attenuation, also known as xe2x80x9crain loss,xe2x80x9d or interference among several UETs, also known as Co-Channel Interference (CCI), may also affect the quality of a signal transmitted from a UET. Each of these conditions, gain roll-off, antenna pointing errors, atmospheric attenuation, and CCI is further discussed below.
1) Gain Roll-Off
The pattern of cells on the surface of the earth is known as the cellular pattern of the satellite communication system. The cellular pattern in a modern satellite communication system may be defined on the surface of the earth such that the maximum gain of a satellite antenna beam is directed toward the center of its assigned cell. The boresight of a satellite antenna beam may be defined as the maximum gain point in the satellite antenna beam, and is typically directed to the center of a cell. The edge of a cell may be defined by determining the angular deviation from the antenna boresight at which the gain of the antenna beam drops to a predetermined value below the maximum gain value, typically at least 3 dB below the maximum gain value. The decrease in antenna beam gain with increasing angular deviation from boresight is known as gain roll-off. In terms of uplink power, a communications signal which is transmitted to the satellite from a UET located at the edge of a cell may be received by the satellite antenna with a gain which is at least 3 dB lower than the gain of a signal which is transmitted from a UET located at the antenna boresight, or center of the cell. Thus, the transmission power level of a terminal located at the edge of a cell must be at least 3 dB higher than that of a terminal located at the center of a cell in order to achieve the same level of performance. In other words, if the edge of a cell is defined as the angle from boresight at which the satellite antenna gain has decayed to 3 dB below the maximum antenna gain at the boresight, a UET at the edge of the cell may need to use a transmission power level 3 dB higher than a UET at the center of the cell in order to compensate for the reduced antenna gain at the edge of the cell. By transmitting at the 3 dB higher transmission power level, the signal from the UET at the edge of the cell may be received at the satellite with a power that is approximately equal to the power of a signal from the UET at the center of the cell. In order to simplify and reduce the cost of uplink components installed on the satellite, it is desirable to maintain a similar received power level for each UET in the cell. Thus, it is desirable to modify the transmission power of each UET in the cell to compensate for any reduction in the antenna gain at each UET resulting from the UET""s position within the cell.
2) Antenna Pointing Errors
In practice, the antenna beams of a cellular communication satellite are generally not directed precisely toward the centers of their assigned cells. Slight mis-orientations of the antenna boresights and deviations from a perfectly circular, zero-inclination satellite orbit give rise to pointing errors. These pointing errors may cause the location of the maximum gain of an antenna beam to deviate from the cell center. Some pointing errors may also cause the maximum gain of an antenna beam pattern to change measurably over the course of a day. In other words, the antenna beam gain distribution across the cell may change with time.
The antenna beam gain at the edge of a cell typically rolls off rapidly as the distance from the center of the cell increases, that is, as the angular deviation from boresight increases. Thus, a pointing error corresponding to only 10% of a cell diameter may cause the antenna beam gain at the edge of a cell to vary by 2 dB or more. Because it is desirable to maintain a similar received power for each UET in the cell, it is desirable to adjust the transmission power of each UET in the cell to compensate for antenna beam pointing errors.
3) Atmospheric Attenuation
Achieving satisfactory communication performance for a signal transmitted from a UET to a satellite generally depends upon receiving a requisite level of signal power at the satellite. That is, each user terminal must transmit a signal with sufficient power to be received. The relationship between the power of the signal transmitted by the terminal and the power of the signal received by the satellite receiver depends in part upon the amount of attenuation of the signal as it passes through the earth""s atmosphere. At Ka-band frequencies, the amount of atmospheric attenuation varies considerably as meteorological parameters and weather patterns change. In particular, the occurrence of rain has a pronounced effect on the attenuation of a Ka-band communication signal. The attenuation of the communication signal is known as rain loss or rain fade, although other meteorological phenomena may also provide attenuation. Such atmospheric conditions and/or weather patterns may change rapidly and may vary among different UETs in a cell depending upon the UET""s position within the cell. Because it is desirable to maintain a similar received power for each UET in the cell, it is desirable to adjust the transmission power of each UET in the cell to compensate for the attenuation experienced by the UET""s signal due to rain loss.
4) Co-Channel Interference
Immediately adjacent cells in a cellular satellite communication system typically use different frequencies for transmitting signals. However, non-adjacent cells may use the same frequency. Such frequency re-use among cells within a cellular pattern serves to reduce the overall frequency bandwidth necessary for the satellite communication system. However, imperfections in satellite antenna beams such as, for example, sidelobe generation, may cause signals transmitted from a UET located in a first cell to be received by a satellite antenna beam which is assigned to receive signals from UETs located in a second cell which uses the same frequency as the first cell. Signals transmitted by UETs located in different cells but using the same frequency may thus interfere with each other, and may cause degraded communication performance. That is, a desired signal received by the satellite from a first UET may be interfered-with by signals from other UETs in other cells using the same frequency as the first UET. The interference from the other UETs may interfere with the desired signal and may adversely affect the performance of the communication system. The interference from other UETs is often referred to as Co-Channel Interference (CCI).
The ratio of the signal power received from the desired UET to the background noise is known as the signal-to-background ratio (SBR). The number of errors in a data signal received from a UET at a satellite (i.e., the error count) may be impacted by the SBR. The background noise may include thermal and other noise sources as well as interference sources such as interference from other UETs using the same frequency. In order for the satellite to receive a signal from a particular UET, the transmission power of the UET must be sufficient to provide at least a certain desired minimum SBR. As the background portion of the SBR increases with increasing CCI, the signal portion of the SBR is also increased to maintain the desired SBR. That is, the UET of interest transmits with increased transmission power to maintain the desired SBR in light of the increasing interference from other UETs. However, increasing the transmission power of the UET of interest raises the background level for the other UETs. The other UETs, also seeking to maintain the desired SBR, in turn respond by raising their transmission powers. The UET of interest may react by further increasing its power, and so on until all terminals in the system are operating at the maximum transmission power. This phenomenon is known as system runaway.
Satellite systems have been proposed that attempt to address the problem of system runaway by establishing a single, constant transmission power level for each UET. These proposed systems contemplated using frequencies in the range of 3 to 12 GHz (C or Ku band). Maintaining a constant power for each UET may be acceptable at Ku or C band frequencies in some cases. However, at higher, Ka-band frequencies (20-30 GHz), for example, attenuation alone may cause the power of the received signals at the satellite to vary over a range of 20 dB or more. A comparable dynamic range would be required of the satellite demodulator, which would have a dramatic impact on system complexity and cost. Additionally, such a system would produce a high degree of CCI and increased power consumption. Because of the high CCI, the maximum tolerable interference level from other UETs would unduly limit the number of UETs that may be used, and system capacity would be needlessly limited. Therefore, it is desirable to maintain satisfactory communication performance (typically, maintain a desired SBR and/or a desired error count) while preventing system runaway.
Additional complexity arises in an uplink power control system with regard to UETs which transmit data intermittently rather than continuously, or whenever a UET first establishes a communication channel for transmission to the satellite. When a UET initiates a transmission, the UET may be forced to send an uplink signal into an attenuation and interference environment substantially unknown to the UET. That is, the UET may not be able to transmit initially with a transmission power that provides the desired SBR while not providing needless CCI to other UETs using the same frequency. If the initial transmission power is set too low, the signal may not be received by the satellite. If the initial transmission power is set too high, it may add a disproportionate amount of CCI and degrade the quality (adversely impact the SNR) of other uplink signals in the system.
U.S. Pat. No. 4,910,792, entitled xe2x80x9cUp-link Power Control in Satellite Communications Systemxe2x80x9d ( the ""792 patent) illustrates one approach for controlling uplink transmission power to compensate for rain attenuation. The ""792 patent illustrates a system including a number of user stations 59, a reference earth station 58, and a satellite 50, identified at column 1, lines 41-43, which is xe2x80x9ca mere repeater of signals, but has no facility to measure the power transmitted from each earth station.xe2x80x9d In operation, the transmission power of a reference signal transmitted from the reference earth station 58 is adjusted so that the received reference signal at the satellite is constant. Each user station 59 transmits a signal which is relayed to the satellite and back to the user station 59. Each of the earth stations 59 then detects the difference between the received reference signal from the reference earth station through the satellite and the level of the received signal with was sent from itself and relayed by the satellite. Each of the earth stations 59 then adjusts its uplink power based on the difference between the signals. That is, the ""792 patent assumes that the reference burst 60 from the reference station 59 is received by the user station 59 with attenuation only on the downlink, while the burst 61 sent from the user station 59 is received at the user station 59 with the attenuation on both the uplink and downlink. Therefore, the difference between the received reference burst signal 60, and the user station burst 61 sent from the user station itself is the attenuation 62 in the uplink, as shown in FIG. 4(b). The system of the ""792 patent applies only to systems employing xe2x80x9cbent pipexe2x80x9d transponders, which are not present in a processing satellite communication system.
U.S. Pat. No. 5,864,547, entitled xe2x80x9cMethod and System for Controlling Uplink Power in a High Data Rate Satellite Communication System Employing On-Board Demodulation and Remodulationxe2x80x9d (the ""547 patent) illustrates another approach for controlling uplink transmission power. In operation, as shown in FIGS. 1 and 5, a downlink error rate of the data in a downlink data stream is determined based on known data bits transmitted by a satellite and received by a receiving terminal 12. An end-to-end error rate of the uplink data stream and the downlink data stream is then determined based on the number of errors in received data transmitted by a first user terminal 11 to the receiving terminal 12. The error rate of the uplink is then indirectly estimated based on the downlink error rate and the end-to-end error rate with reference to a lookup table. Finally, the power of the uplink is controlled based on the indirect estimate of error rate of the uplink. Thus, the ""547 patent relies on an indirect estimate of uplink signal quality using downlink signals. Therefore, errors introduced in the downlink may not be reliably separable from errors introduced in the uplink. The ""547 patent does not determine the uplink error rate directly.
Thus, a need has long existed for a system and method for controlling the uplink power in a satellite communication system. A need has especially existed for such a system and method able to control uplink power in an uplink channel affected by gain roll-off, antenna pointing errors, atmospheric attenuation, and CCI. Additionally, a need has long existed for such a system and method to control initial uplink transmission power. Finally, a need has long existed for a system able to measure an uplink power level or data error rate directly.
A method and system for controlling uplink power in a satellite communication system using error leveling is provided. The uplink power control system for a satellite communication system of a preferred embodiment of the present invention generally comprises a communication satellite and at least one user terminal. The communication satellite may include an error detector for determining the error count in a received data signal transmitted from a user earth terminal (UET) to the communication satellite using a particular channel. The communication satellite may also include a comparator for generating an error indicator signal in response to a comparison of the error count to a predetermined error threshold. The UETs generally include a receiver for receiving the error indicator signal from the comparator, and a controller for controlling the transmit power level of the particular channel being used by the UET in response to the error indicator signal.
The method for controlling the transmit power level of a particular channel assigned to a UET in a satellite communication system of a preferred embodiment of the present invention includes the steps of transmitting an uplink data signal using the channel from the UET to a satellite, receiving the data signal in the particular channel at the satellite, determining an error count for the uplink data signal in the channel using the data signal received from the UET, and comparing the error count to a predetermined error threshold. The method of the present invention may further include generating an error indicator signal for the channel in response to the comparing step, transmitting the error indicator signal to the UET, receiving the error indicator signal at the UET, and controlling the transmit power level of the particular channel in response to the error indicator signal.
The present invention may comprise an individual component of a comprehensive power control system, such as that described in U.S. patent application Ser. No. 09/596,683 (TRW 22-0107), filed Jun. 19, 2000, entitled xe2x80x9cComprehensive System and Method for Uplink Power Control in a Satellite Communication Systemxe2x80x9d.